1. Field of the Invention
This invention relates to electric power-generating devices, such as wind turbines and ocean current turbines, and more particularly to an apparatus for extending and retracting extendable rotors of horizontal axis, axial flow wind or water turbines.
2. Description of the Prior Art
U.S. Pat. No. 3,606,571 of Wood entitled “Stowed Rotor” granted Sep. 20, 1971, describes a stowed rotor mounted atop the fuselage of an airplane. The rotor includes a rotatable housing unit mounted on a vertical shaft and provided with a pair of rotor blade units that telescope into the housing unit. A mechanism is provided to extend and retract the rotor blade units from the housing unit, for the purpose of providing vertical lift during takeoff and vertical landing.
An aircraft may take off vertically or may be landed vertically by extending the rotor blades after imparting operating movement to the housing and rotor blades about a shaft. Rotary motion is imparted to the shaft by motive means not shown. After takeoff, when sufficient altitude is obtained, a conventional power system, such as forward thrust engines, may be employed.
The Wood patent is concerned with a stowed rotor arrangement for producing vertical lift for an aeronautical vehicle. The housing unit is mounted on the vehicle and rotatable about an axis, which is in general alignment with the direction of lift, using a pair of rotor blades telescopically mounted in the housing unit and disposed in generally transverse relation to the axis of rotation of the housing unit.
Wind and water current applications are not concerned with producing vertical lift for an aeronautical vehicle. On the contrary, in wind and current systems the rotors are mounted on a stationary structure and are rotatable about an axis, which is in general alignment with the direction of the wind or water current. In Wood, the housing unit is mounted on the vehicle and rotatable about an axis, which is in general alignment with the direction of lift, not in alignment with the wind or water current. In wind and water current applications the rotors are employed in a fundamentally different way to achieve a fundamentally different result. That is, the rotors are in alignment with the wind or water with the result that the rotors are moved by the current to produce electricity. In Wood, the rotors are in alignment with the direction of lift with the result that the rotors are moved by an engine to produce vertical lift. Wood describes a mechanism for a variable diameter rotor for aerospace applications wherein the rotor is driven by an engine and moves perpendicularly with respect to the flowing medium. Wood does not address the requirements of a wind or ocean current application, wherein the rotors are in alignment with and are driven by a flowing medium and do not move with respect to the flowing medium.
U.S. Pat. No. 3,814,351 of Bielawa entitled “Coaxial Rotor Yaw Control” granted Jun. 4, 1974, discloses coaxial counter-rotating rotors having telescoping blade tip portions which are normally partially extended. The blades of the upper and lower rotors can be differentially extended and retracted to create a resultant net torque between the rotors. The purpose is to provide yaw control by providing telescoping blade tip portions, which are differentially operated by a pilot-operated cable system that extends the tip portions of one rotor while retracting the tip portions of the other rotor.
Each blade is comprised of a hollow spar, which forms the leading edge and is the main strength member of the blade and a tapered trailing edge portion, which completes the airfoil contour of the blade. Each blade has a tip portion of reduced chord which has one end inserted into a cavity in the outboard end of the blade spar in which it is freely slidable. The tip portion is supported by two rollers on the spar, mounted at spaced points along its leading edge on pivots and by rollers mounted on pivots carried by the spar in position to engage the top and bottom tapered surfaces of the tip portion adjacent its trailing edge.
The Bielawa patent does not address problems that arise with respect to an extendable rotor blade system that is fixed with respect to the flowing medium, whether the medium is air or water or any other fluid-flow medium.
The above prior art references describe mechanisms for aerospace applications wherein the rotor is driven by an engine and moves with respect to the flowing medium. These references do not address the requirements of a wind or ocean current applications, wherein the rotor is driven by a flowing medium and does not move with respect to the flowing medium and where durability and fatigue resistance are paramount to the success of such system, and wherein forces acting upon the rotor vary significantly during each revolution.
The mechanisms suggested in the prior art for controlling variable diameter rotors for tilt rotors and aircraft are susceptible to fatigue failures and require extensive maintenance. Wind turbines and ocean current turbines operate in environmental conditions that can quickly degrade the properties of an extension mechanism. The high maintenance requirement translates to higher energy cost, which results in a less competitive renewable energy system.
U.S. Pat. No. 4,180,372 of Lippert, Jr. entitled “Wind Rotor Automatic Air Brake” granted Dec. 25, 1979 discloses a spring-loaded pivoting end plate braking mechanism for a wind rotor. The end plate is hinged such that it is deployed by centrifugal force or a speed change detected by a sensor which controls an actuator to effect the required positioning of the brake plate into the air stream. The brake plate acts as an aerodynamic brake for wind turbines in over-speed conditions. The brake has a stationary portion fixed on the tip of the turbine rotor and a pivoting portion hinged for movement with respect to the fixed portion.
This patent teaches an aerodynamic windmill over-speed limiter which is located at the blade tip with its hinge axis transverse to the rotor blade chord such that the device has maximum effectiveness when deployed for braking and which, in its stowed position, acts to improve the aerodynamics of the rotor blade itself.
Because the brake is located at the rotor blade tip, the drag produced by the brake is at the greatest possible rotor radius such that a maximum torque braking effect is achieved. Also, when the brake plate is deployed, the trailing edge aft of the hinge line is rotated inwardly such that it is interposed over the tip area of the rotor blade. The brake, therefore, not only produces a drag at the maximum possible moment arm but, at the same time, it destroys efficient airflow over a portion of the blade that is normally very effective in driving the rotor.
It is desirable to produce the opposite effect: increasing the length of the rotor blade to improve efficient airflow over the outer extremity of the blade to increase its effectiveness in driving the rotor without introducing drag or braking.
U.S. Pat. No. 4,710,101 to Jamieson entitled “Wind Turbine” granted Dec. 1, 1987, discloses a wind turbine in which movable nose portions are located at or adjacent the leading edge of the blade and at or adjacent the tip of the blade. The nose portions are displaceable longitudinally of the blade, i.e. radially outwardly of the blade, from a normal retracted position. This moveable portion contributes to the lift of the airfoil section, and is moved to an advanced position in which drag is produced, to prevent unwanted increase in the speed of the rotation of the rotor.
The movable portion when in the normal, retracted position, will have little harmful effect on the aerodynamic shape of the airfoil section, the flow lines of the air passing from the movable portion extremely smoothly onto the remainder of the airfoil section.
The leading face of the remainder of the airfoil section has a flat or concave surface to increase the drag effect when the movable portion is in the advanced position. Further to increase the drag effect, bleed passages may lead from the leading faces of the remainder of the airfoil sections, which are exposed when the movable portions are moved to the advanced position. This bleed passages can extend to a major surface of the remainder of the respective airfoil section, to cause air to flow from the leading face to said major surface to cause separation of flow and increase drag. The portion exposed may in fact include part of the operating mechanism of the movable portion, which would even further increase the drag effect.
When the speed of rotation of the rotor reaches a value, which is the maximum value, which can be tolerated, the nose portions move radially outwardly. The nose portions move either under the action of centrifugal force against the return force of springs, or together with assistance from actuators, and the leading faces are exposed. The outward movement of the nose portions will itself cause an effective reshaping of the cross-section of the blades so they do not resemble an airfoil section at all, at the tip of the blade. This destroys lift on a section of the blade where the most power is produced. It will create much more drag on the exposed section, that is the leading face, which may be contoured or roughened to produce maximum drag. The displaced nose sections create drag at a radius beyond the normal position of the tip, where the velocity is higher and the effectiveness is greater.
The present invention is concerned with the opposite effect: increasing the length of the rotor blade to improve efficient airflow over the outer extremity of the blade to increase its effectiveness in driving the rotor without introducing drag or braking.
U.S. Pat. No. 5,630,705 of Eikelenboom entitled “Rotor Construction for Windmill” granted May 20, 1997 discloses a device for converting wind flow energy into mechanical energy. The device has a base construction and a rotor with a horizontal axis mounted on the base. The rotor has a number of elongated rotor blades, which are connected to a rotary support and extend radially therefrom. Each rotor blade or a part thereof is connected to the rotor support by a hinge connection for tilting the longitudinal axis of the rotor blade or part thereof to a predetermined orientation relative to the axis of rotation of the support. A hinge axis of the hinge connection between the rotor blade and the rotary support is directed at an acute angle both to the longitudinal axis of the rotor blade and to the axis of rotation of the support.
The maximum wind-braking area, to be used at relatively low wind speeds, is achieved when the rotor blades are at right angles to the wind direction, while pivoting the rotor blades away in the wind direction and pivoting the rotor blades around their longitudinal axes results in a lower wind-braking area to be used a relatively high wind speeds.
In order to increase the adjustability of the wind-breaking area to the actual wind speed, the rotor blades are formed by a number of elongated rotor blade parts, which are adapted to be placed in a position fully or partially overlapping each other in the lengthwise direction, or essentially in line with each other. For a minimum length of such a rotor blade, the component parts of the rotor blade fully overlap each other. A maximum length of such a rotor blade is achieved if all component rotor blade parts are placed in line with each other.
FIG. 5 of Eikelenboom illustrates an elongated, hollow first rotor blade part that is hingedly connected to an arm. The first rotor blade part contains an elongated, hollow second-rotor-blade part. The second rotor blade part can in turn contain an elongated third rotor blade part. The rotor blade parts can be shifted relative to each other in the lengthwise direction by separate mechanisms including a motor drive, a spindle and a wire cable for each moveable part fitted in the first rotor blade part. The wire is wound on the spindle. The wires can be subjected to both tensile stress and pressure, and a separate wire, spindle, motor arrangement is connected is to the first and second rotor blade parts, respectively, for the purpose of shifting the rotor blade parts in and out relative to each other.
A disadvantage of the device shown FIG. 5 of Eikelenboom is that the first rotor blade into which the second blade part slide must be completely hollow in order to accommodate the shape of the second blade. In modern large-scale turbine the blades are of such a size that reinforcing rib supports are necessary to obtain strength in large-scale wind and water current applications. The cable mechanism itself is not suitable for large scale turbines because the wires must be capable of being subjected to both tensile stress and pressure and such cables are not available for moving heavy objects.
As can be seen from the above descriptions, in the prior art it is known that the length of a blade can be adjusted such that the wind-braking area is varied. A disadvantage of the prior art devices is the number of component parts, which makes the devices complex to build, to service and to repair. Furthermore, prior art devices store an extendable rotor blade within the main blade structure. In order to make room for the extender blade, internal structural support in the main blade must be compromised, weakening the main blade. The chord size of the extender blade is also limited by the space available between spars in the main blade structure.
Commonly assigned U.S. Pat. No. 6,726,439 of Geoffrey F. Deane and Amir S. Mikhail granted Apr. 27, 2004 entitled “Retractable Rotor Blades For Power Generating Wind And Ocean Current Turbines and Means For Operating Below Set Rotor Torque Limits”, discloses a control for extendable rotor blades but does not describe in detail a mechanism for extending and retracting a rotor blade on a wind or water current driven turbine.
What is needed is a mechanism for wind or ocean current turbines which will facilitate extension and retraction of extendible rotor blades and which will be lightweight, easily maintainable, and durable, that will not weaken the main blade structure.
It is desirable that extendable rotor blades be capable of being retrofitted to existing blade structures.
It is also desirable that extendable rotor blades be supported by the ribs and spars that strengthen the main rotor blades.